Sunday, September 14, 2014

New paper debunks the "wet gets wetter, dry gets drier" meme

A new paper published in Nature Geoscience finds the "wet gets wetter, dry gets drier" meme derived from climate models is an oversimplification of climate change not supported by observational data, and that the opposite pattern of "dry gets wetter, and wet gets drier" is almost as likely to occur. According to the authors,

"assessments of observed continental dryness trends yield contradicting results. The concept that dry regions dry out further, whereas wet regions become wetter as the climate warms has been proposed as a simplified summary of expected as well as observed changes over land, although this concept is mostly based on oceanic data.

We find that over about three-quarters of the global land area, robust dryness changes cannot be detected. Only 10.8% of the global land area shows a robust ‘dry gets drier, wet gets wetter’ pattern, compared to 9.5% of global land area with the opposite pattern, that is, dry gets wetter, and wet gets drier. We conclude that aridity changes over land, where the potential for direct socio-economic consequences is highest, have not followed a simple intensification of existing patterns."

The paper joins at least one other finding the "wet gets wetter and dry gets drier" meme is false on local scales, although this belief is still commonly held in the climate science community. American Meteorological Society President Dr. Marshall Shepherd tweeted a few months ago that one of his "toughest challenges" is "explaining to linear thinkers that dry/drier, wet/wetter is expected. They want either or" :

These two papers demonstrate that this meme is a gross oversimplification as about as likely as the reverse to occur, with no change in aridity found in ~80% of global land area observations from 1948-2005. Likewise, the common assumption that dry and wet areas will become more extreme is also premature and potentially false. UPDATE: Another paper published in Geophysical Research Letters finds

"We report a near-zero temporal trend in global mean Precipitation. Unexpectedly we found a reduction in global land Precipitation variance over space and time that was due to a redistribution, where, on average, the dry became wetter while wet became drier."

Thus, at least 3 papers have put the "wet gets wetter, dry gets drier" meme to bed, and the opposite may actually be true [and would be more consistent with natural homeostasis of the hydrological cycle].

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Changes in the hydrological conditions of the land surface have substantial impacts on society1, 2. Yet assessments of observed continental dryness trends yield contradicting results3, 4, 5, 6, 7. The concept that dry regions dry out further, whereas wet regions become wetter as the climate warms has been proposed as a simplified summary of expected8, 9, 10 as well as observed10, 11, 12, 13, 14changes over land, although this concept is mostly based on oceanic data8, 10. Here we present an analysis of more than 300 combinations of various hydrological data sets of historical land dryness changes covering the period from 1948 to 2005. Each combination of data sets is benchmarked against an empirical relationship between evaporation, precipitation and aridity. Those combinations that perform well are used for trend analysis. We find that over about three-quarters of the global land area, robust dryness changes cannot be detected. Only 10.8% of the global land area shows a robust ‘dry gets drier, wet gets wetter’ pattern, compared to 9.5% of global land area with the opposite pattern, that is, dry gets wetter, and wet gets drier. We conclude that aridity changes over land, where the potential for direct socio-economic consequences is highest, have not followed a simple intensification of existing patterns.

With a new analysis of land regions, ETH climate researcher are challenging the general climate change precept that dry regions are getting drier and wet regions are getting wetter. In some regions they are encountering divergent trends.

Based on models and observations, climate scientists have devised a simplified formula to describe one of the consequences of climate change: regions already marked by droughts will continue to dry out in the future climate. Regions that already have a moist climate will experience additional rainfall. In short: dry gets drier; wet gets wetter (DDWW).

However, this formula is less universally valid than previously assumed. This was demonstrated by a team of ETH climate researchers led by Peter Greve, lead author of a study recently published in Nature Geoscience. Traditional analyses use technology that can comprehensively describe climate characteristics above the ocean, but is problematic over land. While this fact was mentioned in said studies, scientific and public discourse has neglected this aspect so far. In their new study, the ETH researchers in the group headed by Sonia Seneviratne's, professor for land-climate dynamics, take into account the specific climatic properties of land surfaces, where the amount of available water is limited when compared with the ocean.

In her analysis, the climate scientists made use of measured data compiled solely on land, such as rainfall, actual evaporation and potential evaporation. The data derived from various sources was combined by Greve and his co-authors -- this allowed them to extract trends in terms of a region's humidity and dryness. Furthermore, the researchers compared data from between 1948 and 1968 and 1984 to 2004.

Half of the surface areas show divergence

The evaluation shows no obvious trend towards a drier or wetter climate across three-quarters of the land are. There are solid trends for the remaining quarter. However, only half of this surface area follows the DDWW principle, i.e. one-eighth of the total landmass, while the trends seem to contradict this rule over the other half.

In some regions, the climate has developed contrary to the general climate formula 'dry gets drier; wet gets wetter' over the past 70 years. (Chart: from Greve et al, 2014)

Some regions which should have become wetter according to the simple DDWW formula have actually become drier in the past -- this includes parts of the Amazon, Central America, tropical Africa and Asia. On the other hand, there are dry areas that have become wetter: parts of Patagonia, central Australia and the Midwestern United States.

Nevertheless, the 'wet gets wetter' rule is largely confirmed for the Eastern United States, Northern Australia and northern Eurasia. 'Dry gets drier' also corresponds to indications in the Sahel region, the Arabian Peninsula and parts of Central Asia and Australia.

However, the DDWW principle does still applies to the oceans. "Our results emphasise how we should not overly rely on simplifying principles to asses past developments in dryness and humidity," Greve explains. This can be misleading, as it cannot do justice to the complexity of the underlying systems.

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The above story is based on materials provided by ETH Zurich. Note: Materials may be edited for content and length.

2 comments:

I put a comment on your link to the Roderick et al paper. Roderick got into problems with his findings on pan evaporation (http://biology-assets.anu.edu.au/CMS/FileUploads/file/Farquhar/272RodericketalPanreviewIIGeogCompass2009_000.pdf), was stopped doing follow up work and moved into another area.